The Tao Te Ching by Lao Tzu

Looked at, but cannot be seen -
That is called the Invisible (yi).
Listened to, but cannot be heard -
That is called the Inaudible (hsi).
Grasped at, but cannot be touched -
That is called the Intangible (wei).
These three elude our inquiries
And hence blend and become One.
Not by its rising, is there light,
Nor by its sinking, is there darkness.
Unceasing, continuous,
It cannot be defined,
And reverts again to the realm of nothingness.
That is why it is called the Form of the Formless,
The Image of Nothingness.
That is why it is called the Elusive:
Meet it and you do not see its face;
Follow it and you do not see its back.
15. The Wise Ones of Old
The wise ones of old had subtle wisdom and depth of understanding,
So profound that they could not be understood.
And because they could not be understood,
Perforce must they be so described:
Cautious, like crossing a wintry stream,
Irresolute, like one fearing danger all around,
Grave, like one acting as guest,
Self-effacing, like ice beginning to melt,
Genuine, like a piece of undressed wood,
Open-minded, like a valley,
And mixing freely, like murky water.
Who can find repose in a muddy world?
By lying still, it becomes clear.
Who can maintain his calm for long?
By activity, it comes back to life.
He who embraces this Tao
Guards against being over-full.
Because he guards against being over-full,
He is beyond wearing out and renewal.
16. Knowing the Eternal Law